One example of the axial piston motor or the axial piston pump is a liquid-pressure rotating device shown in FIG. 17. A liquid-pressure rotating device 1 is configured such that: pistons 10 are attached to respective piston chambers 9a of a cylinder block 9; and the pistons 10 rotate together with the cylinder block 9. At this time, the pistons 10 rotate along a swash plate 12 and also reciprocate. Thus, the volume of a space surrounded by the piston 10 and the piston chamber 9a changes. The liquid-pressure rotating device 1 configured as above serves as a motor or a pump. As performance of such a motor or pump, a maximum rotating speed and an energy conversion efficiency are important. The pistons 10 are important members which determine the performance of the motor or pump.
To rotate the liquid-pressure rotating device 1 at high speed, the weight of the piston 10 needs to be reduced. As shown in FIG. 17, the piston 10 including a hollow portion 10b is used.
According to the piston 10 including the hollow portion 10b, an operating liquid flows into the hollow portion 10b. Therefore, even when the piston 10 is located at a top dead center as shown in an upper side of FIG. 17, the volume (dead volume) of the space surrounded by the piston 10 and the piston chamber 9a becomes large. When suctioning or ejecting the operating liquid, a compression loss generated by compressibility of the operating liquid becomes large.
To realize the weight reduction and the small dead volume, a piston 48 shown in FIGS. 18 and 19 is used (see PTL 1, for example). As shown in FIG. 18, the piston 48 includes a peripheral wall main body portion 49 and a lid portion 50.
The peripheral wall main body portion 49 is a cylindrical member including the hollow portion 10b. An opening portion 51 which communicates with the hollow portion 10b is formed at one end of the peripheral wall main body portion 49.
The lid portion 50 is a circular plate-shaped member including a center hole 52. The lid portion 50 is welded to an inner peripheral surface of the opening portion 51, which is formed at one end of the peripheral wall main body portion 49, to seal the hollow portion 10b. An outer surface of the lid portion 50 is formed as a pressure receiving surface 50a. 
As shown in FIG. 18, a tubular liquid passage portion 53 is formed at a position through which a center line of the piston 48 extends. The tubular liquid passage portion 53 causes outer surfaces of both end portions of the piston 48 to communicate with each other.
Since the piston 48 shown in FIG. 18 includes the hollow portion 10b, the weight reduction of the piston 48 can be realized. In addition, since the hollow portion 10b is sealed by the lid portion 50, the dead volume can be made small.